What reportedly happened: A flight instructor and student pilot were practicing stalls when the airplane experienced a sudden, total loss of engine power. A forced landing ensued.

The post-accident examination of the engine revealed that the left crankcase half idler gear shaft was dislodged. The two idler gear shaft retaining bolts were broken. The idler gear that interconnected the crankshaft and camshaft remained on the idler shaft and was fractured. Both gear shaft retaining bolts were separated. The bolts displayed severe wear through the threaded shanks that removed the majority of the bolt diameters. The lock plate was fractured in several locations and heavily distorted. The bolt holes in the lock plate were severely elongated and enlarged over their original diameter.

While it was likely that loosening of the bolts precipitated the gear failure, no conclusive evidence was uncovered to confirm that scenario. The engine’s most recent annual inspection was performed about four months prior to the accident. At that time, the engine had been operated for 1,226 hours since it was overhauled in 1985, and 639 hours since it was disassembled and reassembled after a propeller strike during 2001.

Probable cause: A failure of the crankshaft idler gear, which resulted in a total loss of engine power while maneuvering.